The present invention relates to porous film materials composed of polytetrafluoroethylene and particularly to asymmetric porous film materials having an unhomogeneous structure that the fiber structure of the porous polytetrafluoroethylene in the surface is different from that in the back and a process for producing the same.
Processes for producing porous polytetrafluoroethylene materials have been described in Japanese Patent Publication Nos. 13560/67 and 18991/76. These processes are characterized by molding a tetrafluoroethylene resin containing a liquid lubricating agent by extruding or rolling or by both to form sheets, rods or tubes and heating thereafter to about 327.degree. C. or more with stretching in at least one direction under an unsintered state. The porous materials produced by these processes have a fiber structure comprising nodules linked by fine filaments, but the state of the nodules and filaments varies somewhat by the stretching ratio, the stretching temperature or the speed at which the materials are stretched. The space surrounded with the filaments and the nodules corresponds with a pore. In general, by increasing the stretching ratio, the length of the filaments increases and the size of the nodules decreases and, consequently, the porosity increases.